Python编程入门到实践 - 笔记（ 5 章）

Posted 2020-10-06

第 5 章练习了以下内容

简单的 if 判断语句

判断字符串是否相等，还是不等

进行数字的大小比较

and，or 比较

检查列表中是否存在指定的元素

if，if-else，if-elif-else 语句写法

if 判断列表是否为空

使用多个列表进行比较判断

这一章的内容也比较简单，感觉和 shell 差不多，但还是多练习吧。

希望路过的大牛指出不足，小弟在此谢过了。

一个简单的 if 判断语句

循环打印 cars 列表中的元素，如果其中的元素等于 bmw，就全部大写打印

否则只是将元素的首字母大写

-------------------------------------------------

cars = [‘audi‘, ‘bmw‘, ‘subaru‘, ‘toyota‘]

for car in cars:  
     if car == ‘bmw‘:    
          print(car.upper())    
     else:    
          print(car.title())

-----------------------------------------------------

Audi  
BMW    
Subaru    
Toyota

判断是否相等

大小写不一样，也会不等

--------------------------

car = ‘Audi‘  
print(car == ‘Audi‘)

--------------------------

True

--------------------------

car = ‘Audi‘  
print(car == ‘audi‘)

---------------------------

False

转换大小写进行比较

------------------------------------

car = ‘Audi‘  
print(car.lower() == ‘audi‘)

------------------------------------

True

检查是否不相等

不过两个值不相等，就打印

-----------------------------------------------

requested_topping = ‘mushrooms‘

if requested_topping != ‘anchovies‘:  
      print("Hold the anchovies!")

------------------------------------------------

Hold the anchovies!

数字的比较

----------------------

age = 18  
print(age == 18)

----------------------

True

进行 if 语句判断，如果两个数字不等，就打印

-----------------------------------------------------------------------------

answer = 17  
if answer != 18:    
     print("That is not the correct answer. Please try again!")

-----------------------------------------------------------------------------

That is not the correct answer. Please try again!

不光可以进行比较是否相等，还可以比较大小

直接在 python 的 IDE 下进行比较是不是看着更方便。哈哈哈

>>> age = 19  
>>> age < 21    
True    
>>> age <= 21    
True    
>>> age > 21    
False    
>>> age >= 21    
False

检查多个条件

and 当两个条件都满足的情况下，打印 True 否则打印 False

>>> age_0 = 22  
>>> age_1 = 18    
>>> age_0 >= 21 and age_1 >=21    
False    
>>> age_1 = 22    
>>> age_0 >= 21 and age_1 >=21    
True

or 至少满足一个条件，打印 True 否则打印 False

>>> age_0 = 22  
>>> age_1 = 18    
>>> age_0 >= 21 or age_1 >= 21    
True    
>>> age_0 = 18    
>>> age_0 >= 21 or age_1 >= 21    
False

检查指定的值是否包含在列表中

----------------------------------------------------------------------------

requested_toppings = [‘mushrooms‘, ‘onions‘, ‘pineapple‘]  
print(‘mushrooms‘ in requested_toppings)    
print(‘pepperoni‘ in requested_toppings)

----------------------------------------------------------------------------

True  
False

给要查找的值指定一个变量并查找，如果不存就打印出来

----------------------------------------------------------------------------

banned_users = [‘andrew‘, ‘carolina‘, ‘david‘]  
user = ‘marie‘

if user not in banned_users:  
     print(user.title() + ", you can post a response if you wish.")

----------------------------------------------------------------------------

Marie, you can post a response if you wish.

简单的 if 语句

设定年龄为19，进行 if 语句判断，如果大于18就打印

---------------------------------------------------

age = 19  
if age >= 18:    
     print("You are old enough to vote!")

----------------------------------------------------

You are old enough to vote!

if-else 语句

设定年龄为 17，与 18 进行比较，如果大于等于 18 就打印 if 下的语句，

否则打印 else 中的语句

----------------------------------------------------------------------------

age = 17  
if age >= 18:    
     print("You are old enough to vote!")    
     print("Have you registered to vote yet?")    
else:    
     print("Sorry, you are too young to vote.")    
     print("Please register to vote as soon as you turn 18!")

------------------------------------------------------------------------------

Sorry, you are too young to vote.  
Please register to vote as soon as you turn 18!

if-elif-else 语句

年龄设定为 17，分别进行判断，小于 4 多少钱，小于 18 多少钱，其他多少钱进行打印

--------------------------------------------------

age = 17  
if age < 4:    
     print("Your admission cost is $0.")    
elif age < 18:    
     print("Your admission cost is $5.")    
else:    
     print("Your admission cost is $10.")

----------------------------------------------------

Your admission cost is $5.

简化一下上面的写法，将判断的值定义一个变量，最后打印

----------------------------------------------------------------

age = 12  
if age < 4:    
     price = 0    
elif age < 18:    
      price = 5    
else:    
     price = 10

print("Your admission cost is $" + str(price) + ".")

-----------------------------------------------------------------

Your admission cost is $5.

使用多个 elif 代码块

判断条件当然不止3个，这时候就用到了多个 elif 代码块

python 并不要求必须有 else 代码块，虽然书里是这么写的，但我作为小白的我还是倔强的认为这个习惯不太好，

所以自作主张不练这个 ’省略代码块‘ 了

----------------------------------------------------------------

age = 12  
if age < 4:    
     price = 0    
elif age < 18:    
     price = 5    
elif age < 65:    
     price = 10    
else:    
     price = 5

print("Your admission cost is $" + str(price) + ".")

----------------------------------------------------------------

Your admission cost is $5.  

检查特殊元素

判断列表中的元素并指定某个元素，进行判断

-------------------------------------------------------------------------------------------

requsested_toppings = [‘mushrooms‘, ‘green peppers‘, ‘extra cheese‘]  
for requsested_topping in requsested_toppings:    
     if requsested_topping == ‘green peppers‘:    
          print("Sorry, we are out of green peppers right now.")    
     else:    
          print("Adding " + requsested_topping + ".")    
print("\nFinished making your pizza!")

-------------------------------------------------------------------------------------------

Adding mushrooms.  
Sorry, we are out of green peppers right now.    
Adding extra cheese.

Finished making your pizza!

在 循环之前先进行一个 if 判断

if requested_toppings 意识是对列表进行判断，列表中至少有一个元素时，返回 True，

现在这个列表为空，返回值为 False，打印 else 代码块中的内容

--------------------------------------------------------------

requested_toppings = []

if requested_toppings:  
     for requested_topping in requested_toppings:    
          print("Adding " + requested_topping + ".")    
     print("\nFinished making your pizza!")    
else:    
      print("Are you sure you want a plain pizza?")

----------------------------------------------------------------

Are you sure you want a plain pizza?

使用多个列表

对 requested_toppings 进行遍历，和 available_toppings 列表中的元素进行比较

-------------------------------------------------------------------------------------

available_toppings = [‘mushrooms‘, ‘olives‘, ‘green peppers‘,  
                      ‘pepperoni‘, ‘pineapple‘, ‘extra cheese‘]    
requested_toppings = [‘mushrooms‘, ‘french fries‘, ‘extra cheese‘]

for requested_topping in requested_toppings:  
     if requested_topping in available_toppings:    
          print("Adding " + requested_topping + ".")    
     else:    
          print("Sorry, we don‘t have " + requested_topping + ".")    
print("\nFinished making your pizza!")

--------------------------------------------------------------------------------------

Adding mushrooms.  
Sorry, we don‘t have french fries.    
Adding extra cheese.

Finished making your pizza!

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